1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a fixing apparatus mounted in a copying apparatus or a printer utilizing a recording technology as an electrophotographic recording method or an electrostatic recording method, and a conveying roller for use in such apparatus.
2. Related Background Art
A copying apparatus or a printer of an electrophotographic process is equipped with a fixing apparatus for heat fixing a toner image formed on a recording material. Such fixing apparatus is available in various types, such as a heat roller type in which a fixing roller heated by an internal halogen lamp and a pressure roller pinch and convey a recording material to execute a heat fixing thereof, an on-demand type (also called film heating type) in which a ceramic heater is contacted with an internal surface of a flexible sleeve (fixing film or fixing belt) formed by a heat-resistant resin or a metal, and a recording material is heated by a fixing nip portion formed by the ceramic heater and a pressure roller, and an electromagnetic induction heating type in which a rotary member itself in contact with a recording material generates heat. In any of these types, a fixing nip portion formed between a fixing roller (or a heater) and a pressure roller pinches and conveys a recording material thereby heat fixing a toner image thereon.
In case of employing an ordinary paper as the recording material, it is necessary to avoid formation of creases in the course of a fixing step. A principal cause of crease formation in the ordinary paper is a shrinkage of paper fibers by an excessive heat supply. As a countermeasure against such phenomenon, there is known a method of employing a fixing roller or a pressure roller (hereinafter such rollers being collectively called a conveying roller) having an inverted crown shape, in which diameter becomes larger from a central portion toward both end portions in the longitudinal direction. For example, a conveying roller, having a metal core of a uniform diameter over the longitudinal direction and an elastic layer provided around the metal core and having a thickness gradually increasing toward the end portions in the longitudinal direction, generates a force of stretching an ordinary paper in the course of the fixing step, thereby suppressing creasing in the ordinary paper.
However, in an ordinary fixing apparatus, in order to form the fixing nip portion, a pressure is applied for example by a spring between an end of the fixing heater (or heater) and an end of the pressure roller, and also between the other end of the fixing roller (or heater) and the other end of the pressure roller. As the nip portion is formed by applying a force in each of both ends of the apparatus, the roller itself shows a certain bending even though the roller has a metal core, so that the fixing nip at the central portion in the longitudinal direction of the roller becomes narrower than in both end portions, thus tending to result in a deficient pressure. Besides, in case of employing a conveying roller having an inverted crown profile as explained above, a further deficiency in pressure tends to be generated in the central portion in the longitudinal direction of the roller, thereby leading to a fixing failure or a toner offsetting.
On the other hand, the pressure roller not internally provided with a heat source tends to show a temperature change by an operation status of the printer. For example in a continuous printing operation of printing on plural recording materials in succession, a large amount of heat is taken away by the recording materials, so that the temperature of the pressure roller does not become very high (for example about 80-90° C.). On the other hand, in an intermittent printing operation in which a preceding recording material and a succeeding recording material have a long interval, a heat amount supplied from the fixing roller (or heater) supplied during such interval becomes larger so that the pressure roller tends to assume a high temperature (for example about 140-150° C.). A fixing property of the toner image is influenced by a heat amount and a pressure given thereto, and a fixing failure and a toner offsetting tend to be generated in a continuous printing operation in which the pressure roller has a low temperature whereby the heat amount given to the recording material and the toner image tends to become low. On the other hand, in an intermittent printing operation, even if the pressure applied to the central portion in the longitudinal direction of the roller is deficient, the fixing failure or the toner offsetting is not generated as in the continuous printing operation as the pressure roller has a high temperature.
Thus, a configuration of merely employing a pressure roller of an inverted crown shape can suppress creases on the recording material but cannot necessarily satisfy a fixing property, and it is very difficult to obtain an ability of suppressing creases and a satisfactory fixing property at the same time. It is therefore conceived to adopt a metal core of a tapered shape in which the diameter becomes gradually smaller from a central portion in the longitudinal direction toward both end portions and to provide an elastic layer in such a manner that the pressure roller has a straight or inverted crown profile at the room temperature (about 10-30° C.) (cf. FIG. 11 and Japanese Patent Application Laid-open No. H09-152803). Also FIG. 9 shows a profile in a thermally expanded state of a pressure roller employing a metal core of a tapered shape and having an inverted crown profile at the room temperature state. Also FIG. 10 shows a profile in a thermally expanded state of a pressure roller employing a metal core of a tapered shape and having a straight profile at the room temperature state.
Such pressure roller, having a large thickness in the elastic layer on both end portions even though having a profile of a straight shape or a little inverted crown shape at the room temperature state, assumes an appropriate inverted crown shape by a thermal expansion of the elastic layer at a high temperature state of the pressure roller (about 140-150° C.), and is therefore capable of suppressing creases that tend to be generated at a high temperature in the ordinary paper. Also showing a smaller inverted crown shape at a temperature of about 80-90° C. where a fixing failure is often generated, it can suppress a decrease in the pressure in the central portion in the longitudinal direction of the roller, whereby it is rendered possible to suppress, to a certain level, a fixing failure or a toner offsetting that is often generated in a temperature range of the pressure roller of 80-90° C.
However, as the metal core of such pressure roller is tapered from a center line in the longitudinal direction toward both ends, the elastic layer in its entire area only shows a monotonous increase in the thickness from the center line in the longitudinal direction toward the both ends. Stated differently, the metal core does not have an area of a uniform diameter in a central portion in the longitudinal direction, and also the elastic layer does not have an area of a uniform thickness in the central portion in the longitudinal direction. Consequently, the elastic layer, when thermally expanded, assumes a V-shaped cross sectional form with a bottom of a recess in the central portion (cf. FIGS. 9 and 10), and is not sufficient in suppressing the fixing failure and the toner offsetting because of presence of such local recessed shape.
Also in a heat fixing apparatus of film heating type, a temperature detecting element for detecting the temperature of a heater is provided on a rear surface of the heater, corresponding to a central portion in the longitudinal direction of the pressure roller, and a current supply control to the heater is executed based on thus detected temperature. Therefore, in case a local concave shape is formed in the central portion of the pressure roller as explained above, the pressure roller takes away, in the central portion, a smaller heat amount from the heater through a fixing film, than in the both end portions, so that the heater temperature tends to become higher in the central portion. For this reason, the temperature detecting element tends to shows a higher detected temperature, thereby maintaining the heater at a temperature lower than an actually desired control temperature and leading to a fixing failure.